Catalytic Oxidation Of Hg0 Research Articles

Structure-activity relationships, product species distribution and the mechanism of effect of multi-component flue gas on Hg0 adsorption and oxidation over CuO/ACs.

A series of Cu-doped activated cokes (CuO/ACs) were synthesized via an impregnation method and applied for the removal of elemental mercury (Hg0). Structure-activity relationships between Hg0 removal and CuO/AC surface characteristics were identified. Hg0 removal over CuO/AC occurs through a combination of physisorption and chemisorption and is mainly dominated by chemisorption. It was found that 1 nm micropores facilitate Hg0 physisorption. Hg0 could weakly adsorb onto an O-terminated crystal layer, whereas strongly adsorb onto Cu-terminated single highly dispersed, clustered and bulk CuO (110) crystal planes via the Mars-Maessen mechanism. Product distributions and mechanisms of Hg0 adsorption and oxidation over the CuO/AC catalyst under multi-component flue gases are also discussed. O2 enhances both physisorption and chemisorption toward Hg0 by 38%. Inhibition of Hg0 removal by SO2 originates from the competitive adsorption and deactivation of CuO cation vacancies, whereas the impact is weakened by O2 through generating 20% of physically adsorbed mercury product species. NO and O2 promote Hg0 chemisorption efficiency by 93% to form Hg(NO3)2. HOCl and/or Cl2 produced by HCl can oxidize 100% of Hg0 to HgCl2, and the catalytic oxidation efficiency is approximately 29%, but O2 slightly lowers the Hg0 catalytic oxidation efficiency by 8%. The affinity ability between various flue gases and Hg0 follows the order O2 < NO < HCl.

Unveiling the electron-acceptor effect in selenium-doped Cu-N-C catalyst with atomically dispersed active sites for boosting Hg0 oxidation

M-N-C catalysts have considerable potential in the fields of catalysis, such as highly polluting gaseous elemental mercury (Hg0) oxidation. However, the direct application of these kinds of catalysts for mercury removal is stymied by their low catalytic efficiency. Heteroatom-doping in M-N-C catalysts has been proposed as a powerful strategy to the promote catalytic of Hg0, but the origin of boosting catalytic activity is still elusive. Herein, it is disclosed that selenium doping induces an obvious electron-acceptor effect for accelerating the Hg0 catalytic oxidation. The model selenium-doped Cu-N-C catalyst with atomically dispersed active sites was verified by various characterization methods, such as SEM-EDS and scanning transmission electron microscopy. The optimal Cu-NC-Se with Cu/Se ratio of 1:1 exhibited the best Hg0 removal performance, over 8-fold enhancement in their Hg0 oxidation capacities than raw selenium-free Cu-NC. At the optimal reaction temperature of 90 °C, Cu-NC-Se achieved an average Hg0 removal rate of 92.44% within 60 min under single pure N2 atmosphere. As for the influence of flue gas components, the presence of O2 promoted the removal of mercury in Cu-NC-Se, whereas CO2, NO and SO2 served as slight inhibitors. Even after five cycles of the experiment, the average mercury oxidization efficiency was still remained at 88.5%, showing that Cu-NC-Se can be reused with good maintenance of the catalytic activity and atomic dispersion of Cu and Se in the structure. The unsaturated Cu sites connected with pyridinic N on the carbon matrix can serve preferential adsorption to Hg0 and doped selenium. Meanwhile, the selenium doping accelerated Hg0 adsorption and accept sufficient electrons for promoting Hg0 oxidation to forming stable HgSe, which is also corroborated by the theoretical analysis results based on density functional theory. This work not only provides an alternative facile synthetic strategy for developing heteroatom-doping atomically dispersed M-N-C catalysts, but also represents a significant advance for deepening the understanding of the Hg0 catalytic oxidation mechanism on M-N-C catalyst.

Effect of Temperature and SO2 on Mercury Removal Performance on a Ce-Cu/Al Catalyst in Low-Temperature Flue Gas

The effects of flue gas temperature and SO2 content on the catalytic oxidation of 2Ce-4Cu/Al catalysts for cooperative control of Hg0 and NO were investigated. The results show that the optimum activation temperature is 200°C. And when the concentration of SO2 in the gas reaches 600ppm, the comprehensive conversion efficiency of Hg0 and NO is optimal, the conversion efficiency remains above 70%. The results of the catalyst analysis before and after the reaction show that both lattice oxygen and chemisorption oxygen are involved in the reaction. Cu and Ce have a cooperative reaction on the catalyst surface. The interconversion between Ce and Cu ions of different values contributes to the formation of surface lattice oxygen, which promotes the Hg0 catalytic oxidation reaction and NH3-SCR reaction.

New frontiers in heterogeneous Hg0 oxidation by O2 over catalytic single-Fe site on boron-vacancy of h-BN: A density functional theory study

The single-atom catalysts (SACs) have shown tremendous potential in heterogeneous catalytic process, because of their high atomic utilization efficiency and controllable characteristic of various active sites. The accurate structure design at atomic scale of SACs brings new opportunities and challenges for the cost-effective Hg0 oxidation and the study in molecularly catalytic mechanism by the oxidant of O2. Herein, a series of single transition metal atom doped (Mn, Fe, Co, Ni, Cu and Zn) on h-BN vacancy (M/V-BN) were well designed and the interaction behaviours were further investigated via Density Functional Theory (DFT) method. The mercury oxidation profiles and mechanism in the existence of O2 had been clarified on the optimized single atom catalytic site. The results show that the six kinds of TM atoms can exist stably on the surface of boron-vacancy (VB-BN) monolayer and have the potential to activate the new two-dimensional material (h-BN) with high surface reactivity. Among them, the most stable configuration Fe/VB-BN exhibited the best affinity to Hg0 with an adsorption energy of -1.697 eV, and the Fe-Hg bond was the shortest of 2.747 Å. The covalent interaction of Fe-Hg bond is demonstrated by the overlaps between Fe d-orbitals and Hg p-orbital. The chemisorption mechanism dominates the adsorption processes of different mercury species (Hg0, HgO, HgCl, HgBr and HgS), with the adsorption energy ranging from -1.2 to -4.2 eV. The Hg0 oxidation over Fe/VB-BN by O2 mainly follows the Eley-Rideal mechanism, and undergoes three key steps, i.e., O2 molecule dissociation, formation of O-Hg-O structure and desorption of oxidized Hg products. O2 was firstly adsorbed on the unsaturated Fe site and strongly activated by the metal center, which contributes to the high Hg0 catalyst oxidation activity. The generation of O-Hg-O structure is the public stage of mercury products desorption. The rate-determining step of Hg0 catalytic oxidation was dependent on Hg2O2 cluster desorption with an energy of 2.91 eV, which was much smaller than the desorption barrier of traditional catalysts. As a whole, the outcomes demonstrate that Fe/VB-BN catalyst is one promising candidate for catalytic oxidation of Hg0 by O2, which enables cleaner coalfired power plant operations.

Co3O4 with ordered pore structure derived from wood vessels for efficient Hg0 oxidation

Catalytic oxidation of Hg 0 to HgO is an efficient way to remove Hg 0 from coal-fired flue gas. The catalyst with ordered pore structure can lower mass transfer resistance resulting in higher Hg 0 oxidation efficiency. Therefore, in the present work, wood vessels were used as sacrificial template to obtain Co 3 O 4 with ordered pore structure. SEM and BET results show that, when the mass concentrations of Co(NO 3 ) 2 ·6H 2 O was 20%, the obtained catalyst (Co 3 O 4 [20%Co(NO 3 ) 2 ]) possesses better pore structure and higher surface area. It will expose more available surface active sites and lower the mass transfer resistance. Furthermore, XPS results prove that Co 3 O 4 [20%Co(NO 3 ) 2 ] has the highest ratio of chemisorbed oxygen which plays an important role in Hg 0 oxidation process. These results lead to a better Hg 0 oxidation efficiency of Co 3 O 4 [20%Co(NO 3 ) 2 ], which is about 90% in the temperature range of 200 to 350 °C. Furthermore, Co 3 O 4 [20%Co(NO 3 ) 2 ] has a stable catalytic activity, and its Hg 0 oxidation efficiency maintains above 90% at 250 °C even after 90 h test. A probable reaction mechanism is deduced by the XPS results of the fresh, used and regenerated catalyst of Co 3 O 4 [20%Co(NO 3 ) 2 ]. Chemisorbed oxygen can react with Hg 0 forming HgO with the reduction of Co 3+ to Co 2+ . And lattice oxygen and gaseous oxygen can supplement the consumption of chemisorbed oxygen to oxidize Co 2+ to Co 3+ .

Experimental and Mechanistic Study of Synergistic Removal of Hg by Evaporation from Desulfurization Wastewater

The flue evaporation of desulfurization wastewater can solve the problem that it is difficult to remove some heavy metal ions and chloride ions by conventional methods. A large amount of chloride ions in desulfurization wastewater can also promote the catalytic oxidation removal of Hg in the flue gas. The migration character of chloride ions in the flue evaporation process of desulfurization wastewater was studied by using the coal-fired thermal state experimental platform. The concentrations of Hg0 and Hg2+ in the flue gas at the inlet and outlet of selective catalytic reduction denitration (SCR), electrostatic precipitator (ESP), and wet desulfurization (WFGD) devices were tested, and the synergistic removal of traditional pollutant removal equipment by flue evaporation of desulfurization wastewater was analyzed. The influence of Hg and the effect of the evaporation of desulfurization wastewater at different positions on the removal of Hg in the flue gas were compared and analyzed, and the catalytic mechanism of Hg on the SCR surface was further revealed. The results show that 10% chloride ions enter the flue gas after the desulfurization wastewater evaporates. The content of chlorine elements and evaporation temperature influence the evaporation of desulfurization wastewater. The mechanism of SCR catalytic oxidation of Hg0 was explored; oxygen atoms have catalytic oxidation effects on Hg0 at different positions in the V2O5 molecule in SCR; and chloride ions can enhance the catalytic oxidation of Hg0 by V2O5. The intermediate product HgCl is generated, which is finally converted into HgCl2. The oxidation efficiency of Hg0 in electrostatic precipitation (ESP) is increased from 3% to 18%, and the removal efficiency of Hg is increased from 5% to 10%. The removal efficiency of Hg2+ in WFGD is basically maintained at approximately 85%. In addition, a small amount of Hg2+ was restored to Hg0 in WFGD. The removal efficiency of Hg0 in the flue gas of evaporative desulfurization wastewater before SCR is 65%, and the removal efficiency of gaseous Hg is 62%. When the evaporative desulfurization wastewater before ESP, the synergistic removal efficiency of Hg0 is 39%, and the gaseous Hg removal efficiency is 39%, and the removal efficiency of Hg is 40%. Evaporation of the desulfurization wastewater before SCR was more conducive to the coordinated removal of Hg by the device.

Simultaneous catalytic oxidation of nitric oxide and elemental mercury over Cu-Fe binary oxide treated by oxygen non-thermal plasma

In this study, non-thermal plasma (NTP) was adopted to improve the catalytic oxidation performance of Cu-Fe binary oxides (CFs) for efficient simultaneous removal of NO and Hg0 from coal combustion flue gas. Sample characterization indicated that the pore structure, surface morphology, and crystalline phases of CF samples were not changed after plasma treatment whereas the contents of lattice oxygen (Ol), Fe3+, and Cu2+ were largely increased. The treated CF samples exhibited far better NO removal performance compared with raw CF over a wide reaction temperature range (150–450 °C). The influences of plasma discharge time, discharge power, discharge atmosphere, and reaction temperature on NO removal performance were analyzed. O2 facilitated Hg0 and NO removal·H2O had adverse effect on Hg0 and NO removal. To some extent, the CF samples exhibited better resistant to sulfur poisoning due to the presence of Fe species. Moreover, the simultaneous removal behavior of Hg0 and NO over the modified CF samples were analyzed. NO facilitated Hg0 removal, but Hg0 had a slight inhibitory impact on NO removal. The optimum reaction temperature of simultaneous removal of NO and Hg0 was 300 °C. Finally, the mechanism responsible for NO/Hg0 removal was revealed, where CuO and Fe2O3 served as active components. During NO removal process, Fe3+, Cu2+, and Ol were first consumed and then recovered due to the existence of O2. Hg0 catalytic oxidation dominated the Hg0 removal process because Hg0 adsorption equilibrium was reached in a short time.

MoS2 quantum dots based MoS2/HKUST-1 composites for the highly efficient catalytic oxidation of elementary mercury

Due to the ever-tightening regulation on mercury emission in recent decades, there is an urgent need to develop novel materials for the removal of elemental mercury at coal-fired power plants. In this study, a series of MoS2 quantum dots (QDs)-based MoS2/HKUST-1 composite materials were prepared. It is found that MoS2 QDs were encapsulated by HKUST-1 and enhanced the crystallinity and specific surface area of HKUST-1. The MoS2/HKUST-1 showed excellent performance in catalytic oxidation of Hg0 as compared with pristine HKUST-1. It is found that surface layer of lattice oxygens is active and participates in Hg0 oxidation, while the consumption of surface oxygens then leads to the formation of oxygen vacancies on the surface. These vacancies are effective in the adsorption and dissociation of O2, which subsequently participates in the oxidation of Hg0. Moreover, the study on the influence of commonly seen gas components, such as SO2, NO, NH3 and H2O, etc., on Hg0 oxidation demonstrated that synergistic effects exist among these gas species. It is found that the presence of NO promotes the oxidation of Hg0 using oxygen as the oxidant.

Mn doped CeO2-MoO3/γ-Al2O3 catalysts for the enhanced adsorption and catalytic oxidation of Hg0 in oxygen atmosphere

A series of Mn-doped CeO2-MoO3/γ-Al2O3 catalysts were synthesized for the adsorption and catalytic oxidation of Hg0 from coal-fired flue gas using oxygen as the oxidizing agent. The results showed that the addition of Mn significantly promoted the adsorption and catalytic performances of CeO2-MoO3/γ-Al2O3 catalysts. The 6 wt% Mn-doped catalysts exhibited the best Hg0 catalytic oxidation efficiency of 86%. The catalysts were characterized to illustrate the structure-activity relationship of Mn-doped catalysts. It is found that the addition of Mn to the CeO2 and MoO3 lattice increases the concentration of oxygen vacancies on the catalyst surface which promotes the catalytic oxidation of mercury. HRTEM results indicate that the incorporation of Mn cracked CeO2 crystal plane with an angle of 105° and may generate more active sites at the interface. Besides, the in-situ DRIFT spectra show that the acidic sites on the Mn-doped catalyst surface were dominated by Brønsted acid sites at lower temperatures which play an essential role in the adsorption of mercury. Overall, This research reveals the effect of Mn additives on the promotion of adsorption capacity and catalytic oxidation efficiency of Hg0, providing a promising approach to widen the effective temperature window of the catalyst.

Catalytic oxidation of Hg0 over Mn-doped CeO2-ZrO2 solid solution and MnOx/CeO2-ZrO2 supported catalysts: Characterization, catalytic activity and SO2 resistance

MnOx-based oxides have been recognized as efficient catalysts for elemental mercury (Hg0) oxidation at low temperature from flue gas. CeO2 or CeO2-ZrO2 solid solution can enhance the catalytic activity of MnOx, so combining manganese and CeO2 or CeO2-ZrO2 is a promising way to obtain more highly effective catalysts. Two kinds of combination modes were considered: one was MnOx loaded on the CeO2-ZrO2 to form a supported catalyst (MnOx/CeO2-ZrO2, Mn/CZ), and the other was Mn-doped into the CeO2-ZrO2 solid solution to generate a new MnOx-CeO2-ZrO2 (CZM) solid solution catalyst. However, it was not yet known which catalyst prepared by the two combination modes has the better Hg0 oxidation performance. To compare their catalytic activities, the two kinds of catalysts with varying Mn mass content were synthesized by the similar method, and the Hg0 catalytic oxidation performances and the effects of SO2 were tested under the same conditions. The catalysts were well characterized by ICP, XRD, Raman, XPS, SEM and N2 adsorption test, and the results suggested that with the same mass fraction of Mn in the catalyst, the BET surface area and the active species concentration (Mn4+ and Oad) of the new MnOx-CeO2-ZrO2 solid solution catalyst were higher than those of the supported catalyst. Activity tests results showed that for the MnOx loaded samples, 18%Mn/CZ exhibited the best activity (64.58%)., while for the Mn doped solid solution, Ce0.525Zr0.175Mn0.3O2 exhibited the highest Hg removal efficiency (89.53%). The mass contents of Mn in 18%Mn/CZ and Ce0.525Zr0.175Mn0.3O2 were similar. The results suggested that the new solid solution MnOx-CeO2-ZrO2 was more promising for Hg0 oxidation. For both types of the catalysts, Mn4+ and surface active oxygen (Oad) were the active species in the Hg0 oxidation reactions. SO2 significantly inhibited the activity of MnOx/CeO2-ZrO2, because SO2 prevented the reaction between Hg0 and the active species (Mn4+ and Oad). The MnOx-CeO2-ZrO2 solid solution exhibited better SO2 resistance, because the SO2 just inhibited the reaction between Hg0 and Oad.

Structure-activity relationship of VOx/TiO2 catalysts for mercury oxidation: A DFT study

Elemental mercury (Hg0), mostly from coal combustion, poses a critical threat to ecosystems and the health of human beings, as it causes several fatal human diseases. Thus, there has been an effort to strengthen regulations around the world. Catalytic oxidation of Hg0 into HgCl2 is considered an economical and practical option, and selective catalytic reduction catalysts such as titania-supported vanadia (VOx/TiO2) have been shown to also oxidize Hg0 to Hg2+. Herein, based on density functional theory (DFT) calculations, we demonstrate the relationship between the coordinative environment of V in the VOx/TiO2 catalyst and the catalytic activity towards Hg0 oxidation, as well as the effect of hydroxylation. We mechanistically estimate the Hg0 oxidation activity of several VOx/TiO2 models using previously reported mechanisms. We also explore the temperature (T)- and pressure (p)-dependent thermodynamic stabilities of the VOx/TiO2 catalyst models by calculating the Gibbs free energy of formation (ΔGform). Finally, the thermodynamic (T, p) conditions that favor high activity of the VOx/TiO2 catalyst are suggested.

Manganese bridge of mercury and oxygen for elemental mercury capture from industrial flue gas in layered Mn/MCM-22 zeolite

Mn-based oxides have been confirmed to possess gaseous elemental mercury (Hg0) uptake performances. One strategy for Mn-based oxides designing was taking full advantage of surface oxygen to bond surface oxidized mercury. In this study, Mn modified layered MCM-22 zeolite were prepared to get a high dispersion of Mn active sites. The composites were characterized and the results showed that the highly dispersed MnOx particles could be readily deposited on the surface and in the channel of layered MCM-22 zeolite. The results showed 5% Mn/MCM-22 zeolite had a Hg0 removal efficiency of 92% and adsorption capacity of 0.32 mg/g at 300 °C under 5% O2. The mechanism of Hg0 removal is mainly associated catalytic oxidation and chemisorption. Mn4+ is the main active site for Hg0 catalytic oxidation to Hg2+, and Mn4+ is only reduced to Mn3+, some of which is re-oxidized to Mn4+ by active oxygen. The Hg2+ combines with oxygen and form HgO absorbed in the pores of Mn/MCM-22 zeolite. The distribution of Hg elemental was better coincident with Mn elemental which further verified the role of Hg-O-Mn, in which Mn played the role of bridge. Therefore, Mn/MCM-22 zeolite is a promising sorbent for Hg0 removal from flue gas.

Charge-distribution modulation of copper ferrite spinel-type catalysts for highly efficient Hg0 oxidation

Hg0 catalytic oxidation is an attractive approach to reduce mercury emissions from industrial activities. However, the rational design of highly active catalysts remains a significant challenge. Herein, the charge distribution modulation strategy was proposed to design novel catalysts: copper ferrite spinel-type catalysts were developed by introducing Cu2+ cations into octahedral sites to form electron-transfer environment. The synthesized catalysts with spinel-type stoichiometry showed superior catalytic performance, and achieved > 90 % Hg0 oxidation efficiency in a wide operation temperature window of 150−300 °C. The superior catalytic performance was closely associated with the mobile-electron environment of copper ferrite. Hg0 oxidation by HCl over copper ferrite followed the Eley-Rideal mechanism, in which physically adsorbed Hg0 reacted with active chlorine species. Density functional theory calculations revealed that octahedral Cu atom is the most active site of Hg0 adsorption on copper ferrite surface. Both direct oxidation pathway (Hg* → HgCl2*) and HgCl-mediated oxidation pathway (Hg* → HgCl* → HgCl2*) played important role in Hg0 oxidation over copper ferrite. HgCl2* formation was identified as the rate-limiting step of Hg0 oxidation. This work would provide a new perspective for the development of admirable catalysts with outstanding Hg0 oxidation performance.

Hg0 catalytic oxidation by HBr over Ce-modified regenerated selective catalytic reduction catalyst

A method of spent catalyst (V-Mo/TiO2) regeneration with simultaneous Ce-modification was designed in this research. The physical and chemical properties, denitrification activity and elemental mercury (Hg0) oxidation ability were studied to select the most effective procedure and Ce content (1 wt%). Compared to the ordinarily regenerated selective catalytic reduction (SCR) catalyst, the Ce-modified regenerated SCR catalyst showed higher catalytic activity for Hg0 oxidation. As for the influence of the flue gas conditions, HBr showed a strong promotion for the Hg0 oxidation on Ce-modified regenerated catalyst, and with the increase of HBr concentration, the catalytic oxidation abilities of both catalysts exhibited an increasing trend. In contrast, NH3 suppressed the catalytic oxidation abilities of both catalysts, particularly the Ce-modified catalyst.

Elucidating the mechanism of Hg0 oxidation by chlorine species over Co3O4 catalyst at molecular level

Co3O4 has been considered as promising active catalyst component for mercury control because of its excellent catalytic and long persistent activity. The heterogeneous mechanisms of Hg0 oxidation by HCl and Cl2 upon Co3O4(1 1 0) surface were studied using density functional theory (DFT) calculations. The results demonstrate that both Hg0 and HgCl2 are chemisorbed upon Co3O4(1 1 0) with the binding energies of −0.74 and −1.43 eV, respectively. HgCl can be molecularly chemisorbed upon Co3O4(1 1 0) and act as intermediate during Hg0 oxidation. HCl and Cl2 dissociate on Co3O4(1 1 0) and convert into active chlorine species. Hg0 catalytic oxidation by HCl and Cl2 on Co3O4(1 1 0) proceeds with Langmuir-Hinshelwood mechanism, in which the chemisorbed Hg0 interacts with the surface active Cl atoms to produce HgCl2. The optimal oxidation pathway includes four steps: (1) Hg0 adsorption, (2) HgCl formation, (3) HgCl2 formation and (4) HgCl2 desorption. The rate-limiting step is HgCl formation with an energy barrier of 0.67 eV. Hg0 oxidation by HCl is thermodynamically and kinetically more favorable than that by Cl2 because the presence of H atoms weakens the interaction between reaction intermediate and Co3O4 surface.

Hg0 oxidation and SO3, Pb0, PbO, PbCl2 and As2O3 adsorption by graphene-based bimetallic catalyst ((Fe,Co)@N-GN): A DFT study

As important parts of simultaneous removal of multiple pollutants in flue gas, catalytic oxidation of Hg0 and adsorption removal of SO3, Pb species and As2O3 on the surface of (Fe,Co)@N-GN catalyst were systematically investigated in this work, using density functional theory. As a result, we found (Fe,Co)@N-GN not only showed great performance on Hg0 oxidation, but also exhibited outstanding removal capacity for SO3, Pb species and As2O3 molecules. Besides, PDOS and EDD analysis were carried out and the result indicated that electron transfer and orbit hybridization played key roles in gas adsorption. In addition, thermodynamics analysis further evidenced (Fe,Co)@N-GN was a qualified sorbent under 550 K, and competitive analysis suggested that the adsorption order of pollution gas was determined by adsorption energy, rather than the volume fraction of corresponding gases in flue gas. We hoped this work can not only lay a foundation for theoretical investigation of Hg0 oxidation, but also provide an important guideline for simultaneous removal of multiple pollutants released from coal-fired power plants.

A new insight into catalytic role of copper sulfate on elemental mercury oxidation: DFT and experimental study

Copper sulfate (CuSO4) is a ubiquitous component in natural minerals and industrial dusts. However, CuSO4 has been believed usually that it has adverse effect on elemental mercury (Hg0) removal from flue gas. Here, we offered a new insight into the effect of crystal water in copper sulfate on Hg0 catalytic oxidation. The structure evolution of copper sulfate by density functional theory (DFT) calculation revealed anhydrous CuSO4 crystal contributing to a better HCl adsorption which provided a precondition for Hg0 oxidation. Experimental results about the catalytic activity sequence of CuSO4 > CuSO4·H2O > CuSO4·5H2O with HCl confirmed the negative effect of crystal water on Hg0 catalytic oxidation. To realize industrial application, supported catalysts CuSO4/α-Al2O3 synthesized by wet impregnation method was employed for elemental mercury (Hg0) oxidation and the results exhibited excellent performance in Hg0 oxidation under 250 °C–400 °C. The Hg0 conversion efficiency was above 97% in the presence of 2 ppm HCl + 6% O2 at 300 °C. Combining computational and X-ray photoelectron spectroscopy (XPS) analyses together, Cu atoms exposure on the surface of anhydrous CuSO4 were the critical active sites for Hg0 oxidation and the reaction obeyed the Eley–Rideal mechanism.

High efficiency of Mn–Ce‐modified TiO2 catalysts for the low‐temperature oxidation of Hg0 under a reducing atmosphere

Manganese‐ and cerium oxide‐modified titania catalysts were prepared by the deposition precipitation for the removal of elemental mercury (Hg0) from simulated yellow phosphorus off‐gas at low temperature. In addition, these catalysts were characterized by X‐ray diffraction, Brunauer–Emmett–Teller measurements, X‐ray photoelectron spectroscopy and field‐emission scanning electron microscope to determine the surface morphology of the obtained compounds and explore their formation mechanism. The results revealed that a Mn–Ce loading and reaction temperature of 10% and 150 °C, respectively, as well as a Mn/Ce molar ratio of 2:1, led to an optimal efficiency for the oxidation of elemental mercury. Furthermore, the effects of flue gas components were investigated. The presence of O2 clearly promoted the oxidation of Hg0. A CO atmosphere did not affect the Hg0 oxidation, when compared with N2, whereas the presence of H2S and water vapor inhibited the oxidation process. Furthermore, the X‐ray photoelectron spectroscopy spectra of Hg 4f revealed that the elemental mercury adsorbed by the catalyst is present as HgO. Finally, the Hg0 catalytic oxidation mechanism was discussed on the basis of the experimental results and characterization analysis.

Cobalt doped ceria for abundant storage of surface active oxygen and efficient elemental mercury oxidation in coal combustion flue gas

Cobalt doped CeO2 (Co-CeO2) prepared by a single-step hydrothermal method was used for Hg0 catalytic oxidation. The catalysts were characterized by scanning electron microscope, transmission electron microscope, X-ray diffraction, Raman spectroscopy, electron paramagnetic resonance, X-ray photoelectron spectroscopy, H2-temperature programmed reduction test, and thermogravimetric measurement. The results show that the exposed planes for the flake-shaped Co-CeO2 mainly were (110) and (100) with low oxygen vacancy formation energies. Abundant oxygen vacancy defects were identified on the Co-CeO2, which further induced plentiful chemisorbed oxygen on the surface. The oxygen storage capacity for the Co-CeO2 was up to 1.43 μmol O2 m−2 at 200 °C, and a large portion of this capacity was for chemisorbed oxygen. Without the introduction of gas-phase O2, Hg0 oxidation probably occurred through a Mars-Maessen mechanism, where active chemisorbed oxygen originated from oxygen vacancy defects reacts with adsorbed Hg0 to form HgO. Active chlorine species was generated from the interactions of chemisorbed oxygen with trace amount gaseous HCl. The active chlorine compensated the slight inhibitive effects of SO2 and H2O, leading to above 90% Hg0 oxidation under simulated low-rank coal burning flue gas atmosphere at a gas hourly space velocity of 160,000 h−1. The abundant active chemisorbed oxygen played important role in and guaranteed an efficient Hg0 oxidation in extremely adverse application environment. These results reveal the role of doping metals on structure-catalytic property relations for CeO2, which opened a new strategy for controlling mercury emission from coal-fired flue gas using CeO2 based catalysts by tuning the morphology and exposed planes.

Heterogeneous reaction mechanism of elemental mercury oxidation by oxygen species over MnO2 catalyst

MnO2-based catalysts have attracted great attention in the field of elemental mercury (Hg0) catalytic oxidation because of their superior catalytic performance and wide temperature window. Quantum chemistry calculations based on density functional theory (DFT) combined with periodic slab models were carried out to investigate the heterogeneous mechanism of Hg0 oxidation by oxygen species (gas-phase O2, chemisorbed oxygen, and lattice oxygen) on MnO2 surface. The results indicate that Hg0 and HgO are chemically adsorbed on MnO2 surface with the adsorption energies of −69.50 and −226.48 kJ/mol, respectively. The adsorption of O2 on MnO2 surface belongs to chemisorption. O2 can decompose on MnO2 surface with an energy barrier of 97.46 kJ/mol to produce two atomic adsorbed oxygen. The perpendicular adsorbed O2 and dissociative adsorbed O2 are more favorable for Hg0 catalytic oxidation than lattice oxygen, and perpendicular adsorbed O2 is the most active oxygen for Hg0 oxidation. The reaction pathway of Hg0 oxidation by perpendicular adsorbed O2 includes three reaction steps: Hg0 → Hg(ads) → HgO(ads) → HgO. The third step (HgO(ads) → HgO) is endothermic by 168.17 kJ/mol with an energy barrier of 179.48 kJ/mol, and it is the rate-limiting step of the whole Hg0 oxidation reaction.